Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Electrorefining of copper includes electrolytically dissolving copper from impure anodes of about 99.7% Cu, and then selectively plating the dissolved copper in pure form onto a cathode. This reaction occurs in a cell containing an electrolyte, which is substantially a mixture of copper sulfate and sulfuric acid.
There are various processes and apparatus for the electrorefining of metal. For the electrowinning of copper, the current industry best practice is toward the production and use of “permanent” stainless steel cathode plates. Such practice is largely based on the original work (and patents) of Jim Perry, et al. of Mount Isa Mines, Queensland, Australia. Such techniques are generically known throughout the industry as ISA PROCESS® technology.
ISA PROCESS® technology (also ISA PROCESS 2000™) is a trade mark of Mount Is a Mines Limited and has been licensed in Australia, Austria, Belgium, Canada, Chile, China, Cyprus, Egypt, England, Germany, India, Indonesia, Iran, Japan, Myanmar, Mexico, Peru, Russia, South Africa, Spain, Sweden, Thailand and USA.
In this process, stainless steel cathode mother plates are immersed in an electrolytic bath with copper anodes. Application of an electric current causes the unrefined base metal from the anode to dissolve into the electrolytic bath and subsequently deposit in a refined form on a cathode blade of the mother plate. The electrolytically deposited copper is then stripped from the blade by first flexing the cathode plate to cause at least part of the copper deposit to separate therefrom, and then wedge stripping or gas blasting the remainder of the copper from the blade.
Such stripping is performed by use of knife-like blades or knife-edge wedges inserted between the steel sheet and the deposited copper at the upper edge of the copper. Alternatively, stripping may be performed by automatically by passing the copper laden cathodes through a hammering station in which the deposited copper is smartly rapped near its upper edge from both sides. This loosens the copper upper edge and stripping is then finished by directing one or more streams of air into the tiny space between the steel and the loosened upper edge of the copper. However, stripping is more preferably effected by the flexion apparatus developed by the Applicants and patented as Australian Patent No. AU 712,612, or by the related method (U.S. Pat. No. 4,840,710).
The cathode mother plate generally consists of a stainless steel blade, and a hanger bar connected to the top edge of the blade to hold and support the cathode in the electrolytic bath.
The ISA PROCESS® employs a system of multiple cells, arranged in series to form practical sections. In the cells, the electrodes, anodic copper and cathodes are connected in parallel.
As an alternative to the ISA PROCESS®, another methodology is the use of starter sheets of higher purity copper, as the cathode substrate upon which the copper is electrodeposited. These starter sheets are produced in special electrolytic cells by a 24-hour electrodeposition of copper onto either hard-rolled copper or titanium blanks.
Preparation of the Starter Sheet Includes Washing, Straightening and Stiffening of the sheet. The sheets are then suspended from rolled copper hanger bars by attached loops of copper strips.
The fundamental difference between the ISA PROCESS® and the conventional starter sheet technology is that the ISA PROCESS® uses a ‘permanent’ reusable cathode blank instead of a non-reusable copper starter sheet.
The key element of the technology is the proprietary design of the ISA PROCESS® cathode plate. The plate itself is fabricated from “316L” stainless steel, welded to a stainless steel rectangular hollow section hanger bar. The hanger bar is encapsulated with electroplated copper for electrical conductivity and corrosion resistance.
Stainless steel is an iron-based metal that contains very low carbon levels (compared to mild steel) and various levels of chromium. Chromium combines with oxygen to form an adherent surface film that resists oxidation. The 316L stainless steel of the ISA PROCESS® cathode plate has an approximate composition of: <0.03% carbon, 16-18.5% chromium, 10-14% nickel, 2-3% molybdenum, <2% manganese, <1% silicon, <0.045% phosphorus, <0.03% sulfur and the balance of iron.
The austenitic 316L is the standard molybdenum-bearing grade. The molybdenum gives 316L excellent overall corrosion resistant properties, particularly higher resistance to pitting and crevice corrosion in acidic environments.
However, selection of the appropriate steel does not, of itself, ensure success. The desired surface adherence characteristics of a cathode plate are that it provides a sufficient tenacity of attachment between the steel sheet and the copper deposited upon it to prevent the copper from peeling or slumping from the steel on its own accord.
To this end, the 316L stainless steel is afforded the “2B” surface finish. The 2B finish is intermediate bright and dull, being a silvery-grey, semi-bright surface produced by cold rolling, softening and descaling, and then final rolling lightly with polished rolls. The result is a semi-bright grey surface that is termed “skinpass-rolled” or “2B” (“B”=bright) and has a surface roughness (Ra) index of between 0.1 and 0.5 μm. 2B steel is often used for process equipment within the food industry when a surface that is easy to keep clean is required.
The smoothness and reflectivity of the surface improves as the material is rolled to thinner and thinner sizes. Any annealing which needs to be done in order to effect the required reduction in gauge, and the final anneal, is effected in a very closely controlled inert atmosphere. Therefore, substantially no oxidation or scaling of the surface occurs and there is no need for additional pickling and passivating.
As used in the ISA PROCESS®, the 2B-finished 316L steel blade is 3.25 mm thick, which is welded to a hollow stainless steel section hanger bar (International Patent Publication number WO 03/062497; US Patent Publication No. US 2005126906). To improve electrical conductivity, the hanger bar is encapsulated with a 2.5 mm thick electroplated copper coating. The vertical edges (Australian Patent No. AU 646,450) are marked with plastic edge strips (International Patent Application number PCT/AU00/00668) to prevent the copper cathode growing around the edges. The bottom edge is masked with a thin film of wax that, whilst preventing the copper enveloping the plate, does not provide a ledge to collect falling anode slimes, which would otherwise contaminate the cathode copper.
Because the manufacture and changing of starter sheets is increasingly costly, refineries operating by these means generally operate two cathode cycles per anode cycle, viz. the starting sheet cathodes are each generally plated with metallic copper for 12 to 14 days before they are removed; a second starter sheet is then inserted between the anodes. Accordingly, the anode cycle is generally of the order of 24 to 28 days. At the end of the cathode cycle the anode scrap is removed, washed and returned to the casting facility for melting and recasting into anodes for further electrorefining cycles.
Although the ISA PROCESS® cathode technology can accommodate variable cathode ages from 5 to 14 days, a 7 day cathode cycle is generally considered ideal, as it fits with the weekly work schedule and shorter working weeks.
The shorter cycle has numerous benefits to cathode quality. When stripped, a single cathode plate produces two single sheets of pure cathode copper. This cathode technology has led to major advancements in the electrode handling systems of copper tank houses. The stainless steel cathode plates offer precision in the straightness and verticality of the stainless steel cathode plate compared with the alternative thin starter sheet. The permanent stainless steel cathode has less chance of trapping falling slimes and other impurities in the cathode deposit during electrolysis. In short, the use of permanent stainless steel cathodes permits process efficiencies otherwise unobtainable employing starter sheets.
Moreover, the use of a stainless steel cathode plate improves current efficiency as fewer short circuits occur and hence less copper modulations are formed. Cathode quality was also improved by the elimination of starter sheet loops.
Cathode chemical quality is exceedingly important with ever more stringent demands (exceeding LME Grade A) being placed on copper rod producers by fine wire drawers. Such quality demands must necessarily start at the copper production source—the cathode copper refineries themselves.
Notwithstanding that the major benefits of the ISA PROCESS® have been to the refiners, tangible secondary benefits have accrued for the end user, who obtains a more consistent, higher quality product. Refining intensity was greatly increased by the benefits of the permanent stainless steel cathode. The inter-electrode gap between the anode/cathode pair could be reduced, thereby increasing the active area for electrolysis per unit length of cell.
Accordingly, the electrical current density for electrolysis may be increased, and today, ISA PROCESS® refineries are operating at around 330 A/m2, whereas conventional starter sheet refineries typically operate at around 240 A/m2.
In-process copper inventory is an important consideration in a refinery operation. In combination, the various ISA PROCESS® efficiencies alluded to above may reduce the in-process copper by the order of 12%—a greatly significant result.